The present invention relates generally to hydrocarbon sensors, and, more particularly, to solid state hydrocarbon sensors having metal and metal oxide electrodes.
Mixed-potential sensors based on oxygen-ion conducting electrolytes have been studied since D. E. Willams et. al. demonstrated the working of a xe2x80x9cPt/YSZ/Auxe2x80x9d CO-sensor operating at Txe2x89xa6400xc2x0 C. Since that time several metal and metal-oxide electrodes have been used to design various mixed-potential sensors for the detection of CO, NOx, and hydrocarbons. Although all these sensors do give a response in the presence of unsaturated hydrocarbons and/or CO, their lack of stability, reproducibility and selectivity have hindered the commercial development of sensors based on this technology.
The first generation of mixed potential sensors (D. E. Willams et. al.) used Gold (Au) and Pt electrodes on a stabilized zirconia electrolyte. The Au electrode in these devices was painted onto the electrolyte. The morphology of this electrode was not easy to reproduce from sensor to sensor, and also the morphology changes with time as the sensor was being operated at elevated temperatures. In order to solve this problem, two other approaches have been tried. The first involved the use of various alloys of Au and other metals with higher melting point than that of Au. The second involved the use of various oxides mixed in with the Au in order to create a cermet electrode.
In the present invention, the Au electrode is replaced with a conductive oxide electrode. The refractory nature of the oxide electrode ensures its morphological stability and the sensor is capable of withstanding temperatures as high as 850xc2x0 C. Moreover, the use of a sintered ceramic pellet (instead of a thin film of oxide) provides excellent control of the electrode area and 3-phase region thus improving the sensor-to-sensor reproducibility.
The mixed-potential that is developed at an electrode/electrolyte interface in the presence of a reducing gas such as CO or hydrocarbons is fixed by the rates of reduction and oxidation of the oxygen and the reducing gas respectively:                                                         1              2                        ⁢                          xe2x80x83                        ⁢                          O              2                                +                      V            2                    +                      2            ⁢                          e              -                                      ↔                  O          0                                    (                  Eqn          .                      xe2x80x83                    ⁢          1                )                                          CO          +                      O            0                          ↔                              CO            2                    +                      V            0                    +                      2            ⁢                          e              -                                                          (                  Eqn          .                      xe2x80x83                    ⁢          2                )            
Both these electrochemical reactions occur at the electrode/electrolyte/gas 3-phase interface and the mixed-potential is that potential V0 at which the rates of these two reactions are exactly equal. That is, the current due to the reduction reaction (Equation 1) equals the current due to the oxidation reaction (Equation 2) when the over-potential of these two reactions equals the mixed potential. The speed (response time) of these sensors will be determined by the time it takes for the two reactions to reach steady state.
In the prior art, a dense YSZ electrolyte is used as the substrate and a metal or metal oxide electrode is deposited on top of this electrolyte. The length of the active 3-phase interface is controlled by the morphology of the electrode. A highly porous electrode results in better gas access and more 3-phase interface, whereas a denser electrode leads to poorer gas access and lesser 3-phase interface. One drawback of this type of arrangement is that the gas has to meander through the pores of a catalytically active material (the electrode) before reaching the 3-phase interface where the reduction and oxidation reactions occur. Hence, the hydrocarbons (or other reducing gases) are heterogeneously oxidized at the metal (or metal oxide) electrode before they reach the 3-phase interface with the electrolyte, with a concomitant loss in sensor sensitivity and response time.
These attributes of the prior art are addressed by the present invention, and various advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention includes a hydrocarbon sensor with an electrolyte body having a first electrolyte surface with a reference electrode depending therefrom and a metal oxide electrode body contained within the electrolyte body and having a first electrode surface coplanar with the first electrolyte surface. The sensor is formed by forming a sintered metal-oxide electrode body and placing the metal-oxide electrode body within an electrolyte powder. The electrolyte powder with the metal-oxide electrode body is pressed to form a pressed electrolyte body containing the metal-oxide electrode body. The electrolyte is removed from an electrolyte surface above the metal-oxide electrode body to expose a metal-oxide electrode surface that is coplanar with the electrolyte surface. The electrolyte body and the metal-oxide electrode body are then sintered to form the hydrocarbon sensor. In a particular aspect of the present invention, the response time of the sensor is improved by sintering at a temperature effective to produce a density less than about 81% of theoretical maximum density.